Electron Paramagnetic Resonance Transitions Research Articles

Magnetic functionalization of ZnO nanoparticles surfaces via optically generated methyl radicals.

The combination of nuclear and electron magnetic resonance techniques, in pulse and continuous wave regimes, is used to unravel the nature and features of the light-induced magnetic state arising at the surface of chemically prepared zinc oxide nanoparticles (NPs) occurring under 120K when subjected to a sub-bandgap (405nm) laser excitation. It is shown that the four-line structure observed around g ∼ 2.00 in the as-grown samples (beside the usual core-defect signal at g ∼ 1.96) arises from surface-located methyl radicals (•CH3), originating from the acetate capped ZnO molecules. By functionalizing the as-grown zinc oxide NPs with deuterated sodium acetate, the •CH3 electron paramagnetic resonance (EPR) signal is replaced by trideuteromethyl (•CD3). For •CH3, •CD3, and core-defect signals, an electron spin echo is detected below ∼100K, allowing for the spin-lattice and spin-spin relaxation-time measurements for each of them. Advanced pulse-EPR techniques reveal the proton or deuteron spin-echo modulation for both radicals and give access to small unresolved superhyperfine couplings between adjacent •CH3. In addition, electron double resonance techniques show that some correlations exist between the different EPR transitions of •CH3. These correlations are discussed as possibly arising from cross-relaxation phenomena between different rotational states of radicals.

High-frequency EPR spectroscopy of paramagnetic manganese centers in GaAs : Mn crystals

High-frequency electron paramagnetic resonance (EPR) is used to study the unique properties of manganese centers in a GaAs : Mn crystal in strong magnetic fields at low temperatures. At frequencies of 94 and 130 GHz, EPR transitions were recorded in the MnGa2+-SH complex, which is a manganese ion with spin S=5/2, which replaces gallium (MnGa2+) and an ionized acceptor (A-) associated via an isotropic antiferromagnetic exchange interaction with a shallow hole (SH) with angular momentum J=3/2. A complex system of energy levels of this complex in a magnetic field and the possibility of accurately determining exchange interactions from EPR spectra are analyzed. Another complex was investigated, in which an ionized acceptor MnGa2+ interacts with a localized hole center in the form of a diamagnetic ion O2- replacing As. This complex, MnGa2+-OAs2-, is characterized by axial symmetry along the <111> axis of the cubic GaAs crystal and an anisotropic EPR spectrum. Due to the high Boltzmann factor, in our studies, the order of the fine structure spin levels of this complex was determined. The effect of the Boltzmann populations of the energy levels on the high-frequency EPR spectra was also demonstrated for the MnGa2+-SH complex. Keywords: high-frequency EPR, GaAs crystal, Mn acceptor, shallow hole, exchange interaction.

High-frequency EPR spectroscopy of paramagnetic manganese centers in GaAs : Mn crystals

High-frequency electron paramagnetic resonance (EPR) is used to study the unique properties of manganese centers in a GaAs : Mn crystal in strong magnetic fields at low temperatures. At frequencies of 94 and 130 GHz, EPR transitions were recorded in the MnGa2+-SH complex, which is a manganese ion with spin S=5/2, which replaces gallium (MnGa2+) and an ionized acceptor (A-) associated via an isotropic antiferromagnetic exchange interaction with a shallow hole (SH) with angular momentum J=3/2. A complex system of energy levels of this complex in a magnetic field and the possibility of accurately determining exchange interactions from EPR spectra are analyzed. Another complex was investigated, in which an ionized acceptor MnGa2+ interacts with a localized hole center in the form of a diamagnetic ion O2- replacing As. This complex, MnGa2+-OAs2-, is characterized by axial symmetry along the <111> axis of the cubic GaAs crystal and an anisotropic EPR spectrum. Due to the high Boltzmann factor, in our studies, the order of the fine structure spin levels of this complex was determined. The effect of the Boltzmann populations of the energy levels on the high-frequency EPR spectra was also demonstrated for the MnGa2+-SH complex. Keywords: high-frequency EPR, GaAs crystal, Mn acceptor, shallow hole, exchange interaction.

Pentacene in 1,3,5-Tri(1-naphtyl)benzene: A Novel Standard for Transient EPR Spectroscopy at Room Temperature

Testing and calibrating an experimental setup with standard samples is an essential aspect of scientific research. Single crystals of pentacene in p-terphenyl are widely used for this purpose in transient electron paramagnetic resonance (EPR) spectroscopy. However, this sample is not without downsides: the crystals need to be grown and the EPR transitions only appear at particular orientations of the crystal with respect to the external magnetic field. An alternative host for pentacene is the glass-forming 1,3,5-tri(1-naphtyl)benzene (TNB). Due to the high glass transition point of TNB, an amorphous glass containing randomly oriented pentacene molecules is obtained at room temperature. Here we demonstrate that pentacene dissolved in TNB gives a typical “powder-like” transient EPR spectrum of the triplet state following pulsed laser excitation. From the two-dimensional data set, it is straightforward to obtain the zero-field splitting parameters and relative populations by spectral simulation as well as the B_{1} field in the microwave resonator. Due to the simplicity of preparation, handling and stability, this system is ideal for adjusting the laser beam with respect to the microwave resonator and for introducing students to transient EPR spectroscopy.

A scanning tunneling microscope capable of electron spin resonance and pump-probe spectroscopy at mK temperature and in vector magnetic field.

In the last decade, detecting spin dynamics at the atomic scale has been enabled by combining techniques such as electron spin resonance (ESR) or pump-probe spectroscopy with scanning tunneling microscopy (STM). Here, we demonstrate an ultra-high vacuum STM operational at milliKelvin (mK) temperatures and in a vector magnetic field capable of both ESR and pump-probe spectroscopy. By implementing GHz compatible cabling, we achieve appreciable RF amplitudes at the junction while maintaining the mK base temperature and high energy resolution. We demonstrate the successful operation of our setup by utilizing two experimental ESR modes (frequency sweep and magnetic field sweep) on an individual TiH molecule on MgO/Ag(100) and extract the effective g-factor. We trace the ESR transitions down to MHz into an unprecedented low frequency band enabled by the mK base temperature. We also implement an all-electrical pump-probe scheme based on waveform sequencing suited for studying dynamics down to the nanoseconds range. We benchmark our system by detecting the spin relaxation time T1 of individual Fe atoms on MgO/Ag(100) and note a field strength and orientation dependent relaxation time.

Высокочастотная ЭПР-спектроскопия парамагнитных центров марганца в кристаллах GaAs : Mn

High-frequency electron paramagnetic resonance (EPR) is used to study the unique properties of manganese centers in a GaAs:Mn crystal in strong magnetic fields at low temperatures. At frequencies of 94 and 130 GHz, EPR transitions were recorded in the MnGa2+ - SH complex, which is a manganese ion with spin S = 5/2, which replaces gallium (MnGa2+) and is an ionized acceptor (A–) associated via an isotropic antiferromagnetic exchange interaction with a shallow hole (SH) with angular momentum J = 3/2. A complex system of energy levels of this complex in a magnetic field and the possibility of accurately determining exchange interactions from EPR spectra are analyzed. Another complex was investigated, in which an ionized acceptor MnGa2+ interacts with a localized hole center in the form of a diamagnetic ion O2– replacing As. This complex, MnGa2+-OAs2-, is characterized by axial symmetry along the <111> axis of the cubic GaAs crystal and an anisotropic EPR spectrum. Due to the high Boltzmann factor, in our studies, the order of the fine structure spin levels of this complex was determined. The effect of the Boltzmann populations of the energy levels on the high-frequency EPR spectra was also demonstrated for the MnGa2+- SH complex.

Magnetically tuning the loss tangent in La(Al1−xFex)O3 using low field electron paramagnetic resonance transitions

In 2012, we demonstrated that microwave loss in practical microwave dielectrics is dominated by electron paramagnetic resonance transitions at cryogenic temperatures. We later used this understanding to develop “smart” materials that switch Fe-doped Al2O3 (εr = 9.8) dielectric ceramics between a low-loss “on state” and a high-loss “off state” at frequencies of ∼12 and ∼19 GHz with a small magnetic field (<100 G). In this report, we extend our work on smart materials to the large dielectric constant (εr = 24) host La(Al1−xFex)O3 so that it can be used in compact resonator and filter designs operating at ∼4 GHz to ∼7 GHz. The Fe3+ ions' zero-field splitting energies are determined by the crystal-field parameters D = 1.55 GHz and E = 0 GHz, along with significant contributions from the higher-order terms, B40(−6.467 MHz) and B43 (160 MHz). These switchable dielectrics may have applications in future communication and Doppler technology.

Dynamic Nuclear Polarization with Vanadium(IV) Metal Centers

Summary Dynamic nuclear polarization (DNP) harnesses the large polarization of electron spins to dramatically increase nuclear magnetic resonance (NMR) sensitivity. This study expands the scope of DNP beyond its traditional focus on hyperpolarizing the solvent network using exogenous polarizing agents (PAs). We introduce 1H DNP with endogenous V4+ centers positioned in a set of vanadyl complexes with tunable V4+-1H distances. We traced the polarization transfer from V4+ to 1H spins, specifically differentiating between direct V4+-1Hs polarization transfer and the 1H spin-diffusion-mediated bulk solvent 1H polarization buildup and illuminated the effect of the V4+-1H distance on these processes. These results deepen our understanding of polarization pathways and expand the catalog of PAs to broad-line transition metals. This study establishes crucial first steps toward employing strategically positioned endogenous paramagnetic metal centers for DNP and the conceptual framework of hyperfine DNP spectroscopy that merges both spatial and chemical diagnosis of target nuclear spins.

EPR Theories for Selection Rules to Observe the Spin Gap

The direct electron paramagnetic resonance (EPR) absorption between the singlet ground state and the triplet excited states of spin gap systems is investigated. Such an absorption, which is forbidden by the conservation of the total spin quantum number in isotropic Hamiltonians, is allowed by some anisotropies; the staggered g-tensor and the Dzyaloshinskii–Moriya interaction. Selection rules in the presence of these interactions were presented in our previous works. We review the selection rules of the EPR transition of the spin gap with some numerical demonstrations and discuss some applications.

Purely Spectroscopic Determination of the Spin Hamiltonian Parameters in High-Spin Six-Coordinated Cobalt(II) Complexes with Large Zero-Field Splitting.

Accurate determination of the spin Hamiltonian parameters in transition-metal complexes with large zero-field splitting (ZFS) is an actual challenge in studying magnetic and spectroscopic properties of high-spin transition metal complexes. Recent critical papers have convincingly shown that previous determinations of these parameters, based only on the magnetic data, have low accuracy and reliability. A combination of X-band electron paramagnetic resonance (EPR) spectroscopy and SQUID magnetometry seems to be a more convincing and accurate approach. However, even in this case, the accuracy of the determination of the spin Hamiltonian parameters is strongly limited. In this work, we propose a purely spectroscopic approach, in which three complementary EPR spectroscopic techniques are used to unambiguously with high accuracy determine the spin Hamiltonian parameters for transition-metal complexes with S = 3/2. The applicability of this approach is demonstrated by analyzing the new quasi-octahedral high-spin Co(II) complex [Co(hfac)2(bpy)] (I). Along with the conventional X-band EPR spectroscopy, we also use such advanced techniques as multi-high-frequency EPR spectroscopy (MHF-EPR) and frequency-domain Fourier-transform THz-EPR (FD-FT THz-EPR). We demonstrate that the experimental data derived from the X-band and MHF-EPR EPR spectra allow determination of the g tensor (gx = 2.388, gy = 2.417, gz = 2.221) and the ZFS rhombicity parameter E/D = 0.158. The axial ZFS parameter D = 37.1 cm-1 is measured for I with the aid of FD-FT THZ-EPR spectroscopy, which is able to detect the high-energy EPR transition between the two Kramers doublets. CASSCF/NEVPT2 quantum-chemical calculations of magnetic parameters and magnetic direct current (dc) measurements are performed as well as testing options, and the results obtained in these ways are in good agreement with those derived using the proposed spectroscopic approach.

The triplet mechanism of electron spin polarization in moderately coupled triplet-doublet rigid complexes as a source of the enhanced +1/2 ↔ -1/2 transitions.

The light-induced electron spin polarization generated in the excited quartet and doublet states of a system consisting of a chromophore with an attached radical is investigated theoretically. Excitation of the chromophore and subsequent relaxation leads to a coupled triplet-doublet spin system. In many such systems, the electronic coupling between the triplet and doublet spins is expected to be strong enough to split the spin system into so-called trip-doublet and trip-quartet states but sufficiently weak that it does not promote significant mixing between the sing-doublet and trip-doublet states. In such moderately coupled systems, the sing-doublet can relax to the trip-doublet and trip-quartet states by spin-orbit coupling mediated intersystem crossing within the chromophore. An analytical expression is derived for the intensity of the polarization generated by this mechanism for the ms = +1/2 ↔ -1/2 electron paramagnetic resonance transitions of the trip-doublet and trip-quartet states. The expression shows that the intensity and sign of the polarization depend strongly on ratio jω = 3J/ω0 between the triplet-doublet exchange interaction J and the Zeeman energy ω0. The polarization becomes undefined when jω = 1 and when jω = 2 because level-anticrossings between the trip-doublet and trip-quartet sublevels occur. The sign of the polarization is also found to change above and below these values. Thus, for such moderately coupled systems, the sign of the polarization and its Zeeman energy dependence can be used to estimate the magnitude of the exchange coupling.

Electronic structure of non-KramersTb3+centers in garnet crystals and evidence of their energy and spin transfer toCe3+emitters

The electronic structure of non-Kramers ${\mathrm{Tb}}^{3+}$ centers in single crystals of yttrium aluminum garnet (YAG) was studied using a new-generation high-frequency magnetic resonance spectrometer that allows measurements of electron paramagnetic resonance (EPR), electron spin echo (ESE), the photon echo on hyperfine components of Stark levels in zero magnetic field, and optically detected magnetic resonance (ODMR). It was created on the basis of a highly stable microwave bridge operating in the near-terahertz region (0.094 or 0.130 THz), both in continuous-wave (cw) and pulse modes, a nonresonant system for supplying microwave power to a sample, and a cryogen-free magneto-optical cryostat. EPR, ESE, and cw measurements at two frequencies allowed us to reliably and with high accuracy determine the parameters of ${\mathrm{Tb}}^{3+}$ centers that occupy yttrium dodecahedral positions in YAG: ${g}_{||}=15.8\ifmmode\pm\else\textpm\fi{}0.2$, ${g}_{\ensuremath{\perp}}\ensuremath{\approx}0$ (|| corresponds to one of the $\ensuremath{\langle}100\ensuremath{\rangle}$ crystal axes), the zero-field splitting $\mathrm{\ensuremath{\Delta}}=2.705\ifmmode\pm\else\textpm\fi{}0.005\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, and the hyperfine interaction constant $A=0.197\ifmmode\pm\else\textpm\fi{}0.005\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$. In addition to the ${\mathrm{Tb}}^{3+}$ centers in a regular environment, EPR spectra with lower intensity and a resolved hyperfine structure were found for at least three types of additional ${\mathrm{Tb}}^{3+}$ centers. They have symmetry, $g$ factors, and hyperfine interaction constants which are close to those of the main ${\mathrm{Tb}}^{3+}$ centers but differ in the zero-field splitting parameters \ensuremath{\Delta} that strongly depend on the crystal field. They were ascribed to ${\mathrm{Tb}}^{3+}$ centers with nearby antisite ${\mathrm{Y}}_{\mathrm{Al}}$ defects. Similarity with several types of ${\mathrm{Ce}}^{3+}$ centers in YAG crystals, the types, and relative concentrations of antisite defects are discussed. For one of the additional ${\mathrm{Tb}}^{3+}$ centers, splitting of the energy levels in the zero field turned out to be close to an energy of 94 GHz microwave quanta, and intense echo signals were observed in weak magnetic fields and in the zero field corresponding to the EPR transitions between the hyperfine components of the ${\mathrm{Tb}}^{3+}$ spin levels. ODMR spectra of ${\mathrm{Tb}}^{3+}$ centers in YAG crystals, containing Ce and Tb, were obtained by monitoring the intensity of the ${\mathrm{Ce}}^{3+}$ luminescence, which implies the transfer of energy from ${\mathrm{Tb}}^{3+}$ ions to ${\mathrm{Ce}}^{3+}$ ions with conservation of spin. This result seems to be important, since these systems are of interest for quantum communication and computations. Taking into account that cerium does not have isotopes with a nuclear spin and that terbium has 100% $^{159}\mathrm{Tb}$ having nuclear spin $I=3/2$ and a sufficiently large nuclear magnetic moment, the ${\mathrm{Tb}}^{3+}\ensuremath{-}{\mathrm{Ce}}^{3+}$ system in garnet crystals can be promising for coherent information processing. ${\mathrm{Tb}}^{3+}$ ions can play a role of qubits and ${\mathrm{Ce}}^{3+}$ ion with a lifetime of about 50 ns, and almost unity quantum efficiency of optical transitions can be used as a single-ion readout arrangement. The prospects for using this system as hardware for quantum communication and computations are discussed.

Eu3+ ion distribution in oxyfluoride glass nanocomposites

Assessment of activator distribution in lanthanide doped nanocomposites is an important and challenging task. Oxyfluoride glass ceramics have been chosen as a model system to characterize incorporation efficiency of Eu3+ ions in CaF2 nanocrystals using a combination of X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) techniques. Lattice constant changes induced by size mismatch of Eu3+ and Ca2+ ions have been used to evaluate Eu3+ content in CaF2. The distortion of CaF2 lattice due to incorporation of europium ions can be detected from EPR investigations via Gd3+ paramagnetic probe spectra. It is shown that ratio of different symmetry CaF2:Gd3+ centres and linewidth of EPR transitions can be used to monitor europium content in the crystalline phase of glass ceramics.

Unraveling the Redox Couples of VIII/VIV Mixed-Valent Na3V2(PO4)2O1.6F1.4 Cathode by Parallel-Mode EPR and In Situ/Ex Situ NMR

We herein present a study on the sodiation/desodiation behavior of Na3V2(PO4)2O1.6F1.4 cathode with I4/mmm tetragonal space group by the combination of dual-mode electron paramagnetic resonance (EPR) and in situ/ex situ solid-state nuclear magnetic resonance (SSNMR) techniques. The complementary parallel/perpendicular mode EPR, in situ 23Na NMR, 23Na/51V MAS NMR, and two-dimensional 23Na MQMAS NMR experiments substantiate that (i) both VIV and VIII are electrochemically active in this material, with the former transforming to VV, whereas the latter transforming to VIV; (ii) VIV continually converts to VV during the whole charge process, whereas VIII mostly reacts at the higher-potential charge plateau region; (iii) the Na+ extraction/intercalation process of Na3V2(PO4)2O1.6F1.4 is a widespread solid-solution mechanism; (iv) electronic spin hopping might occur locally between VIII and VV ions generated during the electrochemical reaction, resulting in the inconsecutive shift in 23Na resonance. This study s...

Exploring the behavior of electron-hole pairs in working organic light emitting diodes

Although the conversion reactions of electron-hole (e-h) pairs into emissive excitons are essential in the operation process of organic light emitting diodes (OLEDs), experimental research on behaviors of e-h pairs in operating OLEDs has not been developed so far. Here, three types of magnetic-resonance techniques that detect changes in current, photoluminescence intensity, and electroluminescence (EL) intensity induced by electron spin-resonance transitions have been applied to a working OLED as a function of operation bias. Combined use of these techniques reveals that e-h pairs exist in the OLED with completely different bias dependence from carriers and excitons. It is shown that the EL process is classified into three regions depending on the bias: pair-accumulation, pair-dissociation/recombination, and non-pair-formation regions.

Switching microwave dielectric resonators from a high-Q on state to an off state using low-field electron paramagnetic resonance transitions

We show that the application of a small (<75 G) magnetic field can be used to switch a low-loss microwave dielectric to a high-loss material. This enables high performing microwave dielectric resonators and filters to be switched between a high-Q on state and a low-Q off state electronically. In this study, the absorption mechanism inducing the low Q state is from electron paramagnetic resonant transitions of an Fe3+ state in a host Al2O3 dielectric. Our modeling efforts show that it is possible to further optimize the performance of these switchable devices through a material selection process, which will entail the choice of host lattice and the J > ½ impurity and its concentration.

Rescaling of 2D ESEEM Data as a Tool for Inverse Problem Solving

A rescaling procedure is proposed for electron spin echo envelope modulation spectra observed at several electron paramagnetic resonance transitions. Analytical expressions describing the relations between the rescaled frequencies and hyperfine and quadrupole parameters of the remote nucleus are obtained. The dependences of the rescaled data on the external magnetic field and spin projections of the ion nucleus and the remote nuclei are used to derive the parameters of the nuclear state in the crystals Y2SiO5 and YVO4 doped by ion Nd3+.

Exploring the Limits of Overhauser Dynamic Nuclear Polarization (O-DNP) for Portable Magnetic Resonance Detection of Low γ Nuclei

Nuclear magnetic resonance (NMR) spectroscopy in portable, permanent magnet-based spectrometers is primarily limited to nuclei with higher gyromagnetic ratio, γ, such as 1H, 19F, and 31P due to the limited field strength achievable in these systems. Overhauser effect dynamic nuclear polarization (O-DNP), which transfers polarization from an unpaired electron to a nucleus by saturating an electron paramagnetic resonance transition with an oscillating radio frequency magnetic field, B1e, can increase the polarization of low γ nuclei by hundreds or even thousands, enabling detection in a portable system. We have investigated the potential for O-DNP to enhance signals using (4-amino-2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO hereafter) as a source of unpaired electrons in a homebuilt ultra-low field (ULF) O-DNP-NMR spectrometer. We have found, in general, that larger concentrations of TEMPO are required for effective O-DNP with low γ nuclei, which has a number of important effects. Spin exchange effects cause the EPR lines to overlap and ultimately merge at high concentrations of TEMPO, fundamentally increasing the maximum possible enhancement, while the electron–electron dipolar interaction reduces both longitudinal and transverse relaxation times for the electrons, dramatically increasing the required B1e strength. The relationship between TEMPO concentration, B1e magnitude and O-DNP enhancement is quantified, and strategies for achieving these fields are discussed.

Small Gd(III) Tags for Gd(III)-Gd(III) Distance Measurements in Proteins by EPR Spectroscopy.

The C7-Gd and C8-Gd tags are compact hydrophilic cyclen-based lanthanide tags for conjugation to cysteine residues in proteins. The tags are enantiomers, which differ in the configuration of the 2-hydroxylpropyl pendant arms coordinating the lanthanide ion. Here, we report the electron paramagnetic resonance (EPR) performance of the C7-Gd ( S configuration) and C8-Gd ( R configuration) tags loaded with Gd(III) on two mutants of the homodimeric ERp29 protein. The W-band EPR spectra were found to differ between the tags in the free state and after conjugation to the protein. In addition, the spectra were sensitive to the labeling position, which may originate from an environment-dependent charge density on the Gd(III)-coordinating oxygens. This is in agreement with previous NMR experiments with different lanthanide ions, which suggested sensitivity to H-bonding. W-band 1H-ENDOR (electron-electron double resonance) experiments detected effects from orientation selection in the central transition, due to a relatively narrow distribution in the ZFS parameters as indicated by simulations. In contrast, the distance distributions derived from DEER (double electron-electron resonance) measurements were insensitive to the R or S configuration of the tags and did not exhibit any orientation selection effects. The DEER measurements faithfully reflected the different widths of the distance distributions at the different protein sites in agreement with previous DEER measurements using other Gd(III) tags. Due to their small size, short tether to the protein, and a broad central EPR transition, the C7-Gd and C8-Gd tags are attractive Gd(III) tags for measurements of relatively short (<4 nm) distances by EPR spectroscopy.

A table-top PXI based low-field spectrometer for solution dynamic nuclear polarization.

We present the development of a portable dynamic nuclear polarization (DNP) instrument based on the PCI eXtensions for Instrumentation platform. The main purpose of the instrument is for study of 1 H polarization enhancements in solution through the Overhauser mechanism at low magnetic fields. A DNP probe set was constructed for use at 6.7mT, using a modified Alderman-Grant resonator at 241MHz for saturation of the electron transition. The solenoid for detection of the enhanced 1 H signal at 288kHz was constructed with Litz wire. The largest observed 1 H enhancements (ε) at 6.7mT for 14 N-CTPO radical in air saturated aqueous solution was ε~65. A concentration dependence of the enhancement is observed, with maximum ε at 5.5mM. A low resonator efficiency for saturation of the electron paramagnetic resonance transition results in a decrease in ε for the 10.3mM sample. At high incident powers (42W) and long pump times, capacitor heating effects can also decrease the enhancement. The core unit and program described here could be easily adopted for multi-frequency DNP work, depending on available main magnets and selection of the "plug and play" arbitrary waveform generator, digitizer, and radiofrequency synthesizer PCI eXtensions for Instrumentatione cards.